1. Technical Field
This invention relates to a technique for reducing aberrations in optical systems, and more particularly to a simple and compact optical element employing the combination of a grating and a prism.
2. Discussion
The prism has long been one of the very basic optical components in many optical systems. It is a very fundamental optical component for many applications such as altering beam direction, zooming, correcting anamorphic aberration and correcting line of sight errors. The applications also include infrared detection, head-up display (HUD), helmet mounted display (HMD), laser radar, commercial cinematography, etc. Unfortunately, the generic dispersion characteristics of a prism limits its applications. That is, because of the variation in index of refraction and dispersion with wavelength, prisms are often not useful in applications involving a wide spectral range of optical radiation. These dispersion characteristics result in optical chromatic aberrations, that is, a departure of the image-forming system from the ideal behavior occurring when a beam passes through a system. In particular, prisms are prone to chromatic aberrations which results from the variation in the index of refraction with wavelength. Chromatic aberration will severely degrade the image quality of an optical system.
However, aberration of an optical component has both a positive and a negative contribution, and if these contributions are balanced, the total aberration of the system can be tolerably small. For example, achromatic and apochromatic prisms take advantage of this approach. Achromatic prisms, which correct for two wavelengths, consist of a pair of prisms with different dispersion coefficients and different prism angles. Apochromatic prisms, which correct for three wavelengths, usually employ two pairs of prisms with at least three different glass materials. While apochromatic prisms attempt to balance the positive and negative contributions to chromatic aberration they still have a number of drawbacks. For example, the residual chromatic aberration is generally larger than desired. Also, both achromatic and apochromatic prisms, because of the different prism components and materials, are generally bulky, complicated and expensive.
Thus it would be desirable to provide achromatic prisms and apochromatic prisms with a simplified construction. It would also be desirable to provide achromatic and apochromatic prisms having improved performance with extremely small residual chromatic and anamorphic aberrations.